Various methods and systems are known in the printing machine industry, capable of being integrated with a printing machine and to check printed products as they pass rapidly through a printing machine, typically a rotary printing machine; and, upon such inspection, to derive comparison signals between actual outputs obtained from sensors sensing the printed products and comparing those output signals with set or desired signals, to derive comparison signals for controlling, for example, variously colored inkers of inking systems, to control the register of printing stations, for example printing in different colors, and, in general, to examine and check the printed output for quality and for errors and mistakes. Sensors responsive to the printed products usually are surface sensors, capable of monitoring small or comparatively large surface areas of the printed product. Such sensors may, for example, be video cameras or line-sensors, checking the product line-by-line, for example an elongated camera apparatus. Area-type sensors have some disadvantages. In order to check a moving web, a flash arrangement must be used to "freeze" the image to be checked, so that blurred zones, due to movement of the substrate on which the image is printed, are avoided. The entire area which is being checked must be within the depth of field range of the camera equipment. This requires either large distances between the camera and the printed subject matter or the area which can be checked while maintaining sharpness is restricted. This technology, thus, is used only for macroscopic testing of a relatively small section of imaged printed material. Basically, this system is suitable for determining register as a function of a plurality of printed line or stripe elements in predetermined distances from each other, see for example the German Patent Disclosure Document DE-OS 36 25 449.
The referenced U.S. Pat. No. 4,488,808 discloses a system and a method which is intended to measure and characterize the quality of printed products during printing operation. Reflection of imaged elements, subdivided from the entire printed image or page, is sensed in a line-by-line sensing arrangement, to obtain electrical signals which are then compared with predetermined set or desired signal values. The sensor can operate continuously; it is only necessary to restrict the reception range of the sensor to a narrow region which just can accept one line for sharp reproduction from that region or line. High printing speeds require very short exposure times, which, in turn, requires high light intensity in order to provide the required light to completely control the sensors. The sensors can store the information of only one line and the termination of illumination must be so arranged that the just illuminated line can be completely read out from the sensor. The read-out frequency from sensors is limited, however, which in turn limits the maximum frequency of scanning of lines. As a consequence, the resolution of test or printed or subject matter is limited since the speed of movement of the printed subject matter is predetermined, by the operating speed of the printing machine. High operating speeds of printing machines permit digitizing of measured values to a maximum extent of eight bits, that is, 256 steps. This is not enough for high-quality optical evaluation. The system of the U.S. Pat. No. 4,488,808, thus, does not entirely overcome the disadvantages of the testing technology as heretofore known.